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Laboratory of Genetics and Biochemistry, Department of Aquatic Biosciences, Tokyo University of Fisheries, Tokyo, Japan
| Abstract |
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2030%. The positions of cysteine residues that
are important for disulfide bonds were conserved with respect to those
in mammalian TNF-
. The Japanese flounder TNF gene has a length of
2 kbp and consists of four exons and three introns. The positions of
the exon-intron junction positions of Japanese flounder TNF gene are
similar to those of human TNF-
. However, the length of the first
intron of Japanese flounder is much shorter than that of the human
TNF-
gene. There are simple CA or AT dinucleotide repeats in the
5'-upstream and 3'-downstream regions of the Japanese flounder TNF
gene. Southern blot hybridization indicted that Japanese flounder TNF
exists as a single copy. Expression of Japanese flounder TNF mRNA is
greatly induced after stimulation of PBLs with LPS, Con A, or PMA.
These results indicated that Japanese flounder TNF is more like
mammalian TNF-
than mammalian lymphotoxin-
, with respect to its
gene structure, length of amino acid sequence, number and position of
cysteine residues, and regulation of gene
expression. | Introduction |
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is a 17-kDa protein that is synthesized by
different cell types upon stimulation with endotoxin, inflammatory
mediators, or cytokines such as IL-1 and, in an autocrine manner, upon
stimulation with TNF itself (1, 2, 3, 4). The biological effect
of TNF-
may vary depending on the relative concentration, the
duration of cell exposure, and the presence of other mediators, and
plays roles in immune and inflammatory responses and in the
pathogenesis of many diseases (1, 2, 3, 4).
There are also several TNF-like proteins, which together are referred
to as the TNF ligand superfamily. The members of the TNF ligand
superfamily (TNF-
, lymphotoxin-
(TNF-ß), lymphotoxin-ß, CD27
ligand, CD30 ligand, CD40 ligand, Fas ligand, OX40 ligand, and TRAIL)
share common biological activities, but some properties are shared by
only some ligands, whereas others are unique. The biosynthesis of TNF
family proteins is largely regulated at the posttranscriptional level
(5). Bacterial endotoxin, which strongly induces
TNF-
production, thus seems to elicit at least some of its effects
by altering macrophages. It has previously been shown that a
3'-untranslated TTATTTAT element that is present in numerous cytokine
genes and protooncogenes is capable of repressing the translation of
mRNA molecules in which it occurs (6).
The nucleotide sequences of the cDNA and genes encoding TNF-
have
been reported for several mammalian species (7, 8, 9, 10, 11, 12, 13, 14). Human
secreted-type mature TNF-
is a nonglycosylated protein of 17 kDa
with a length of 157 amino acid residues and forms dimers and trimers
(4). The gene of human TNF-
has a length of
3 kbp
and is interrupted by three introns (15). The structure of
human TNF-ß is different from that of TNF-
(15). The
protein coding regions of the human TNF-ß gene are separated into
three parts (15).
Thus, mammalian TNF family proteins have been well characterized. However, no information is available on TNF proteins in fish. To understand the role and mechanism of TNF in the immune system in fish, it is essential to identify and characterize the TNF. In this study, we isolated a cDNA and gene for the TNF from Japanese flounder Paralychthys olivaceus.
| Materials and Methods |
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Peripheral blood samples were taken from a single homocloned
Japanese flounder P. olivaceus (16). Leukocytes
were isolated by centrifugation, at 400 x g for 20
min, with Percoll solution (1.072 g/ml). Leukocytes were cultured in
RPMI 1640 containing Con A (70 µg/ml) and PMA (0.35 µg/ml) and were
sampled after 1, 2, and 3 h. mRNA was isolated using a micro mRNA
purification kit (Amersham Pharmacia Biotech, Piscataway, NJ). The
purified mRNAs of three different time periods were pooled and used to
construct a cDNA library. cDNA was synthesized using a cDNA synthesis
kit (Amersham Pharmacia Biotech) with an oligo(dT) primer. The cDNA
library was constructed in
ZAPII vectors (Stratagene, La Jolla, CA)
according to the instructions of the manufacturer.
Screening of cDNA
We conducted an expressed sequence tag analysis of the cDNA
library prepared from Con A/PMA-treated leukocytes. Conversion of the
recombinant
ZAPIIs into the pBluescript plasmid was conducted by in
vivo excision according to the protocol of the manufacturer
(Stratagene). After conversion of phage clones into plasmids, we
randomly selected clones from the library and sequenced them. cDNA
clones were sequenced using ThermoSequenase (Amersham Pharmacia
Biotech) with M13 forward and/or M13 reverse primers and an automated
DNA sequencer LC4200 (Li-Cor, Lincoln, NE). Each determined sequence
was compared with all sequences available in DDBJ/EMBL/GenBank using
the BLAST version 2.0 (17, 18)
(http://www.ncbi.nlm.nih.gov).
Screening of gene
Previously, we constructed and arrayed a genomic bacterial
artificial chromosome
(BAC)3 library
(19). The average insert size of clones in this library is
165 kbp. The arrayed genomic BAC clones were screened for a TNF gene
by using a TNF cDNA as a DNA probe. Hybridization was done as
previously reported (19). BAC DNAs were isolated by the
alkaline lysis method and then digested with EcoRI and
subcloned into the pUC119 plasmid vector. The subclones were screened
by the colony-hybridization method (20). The genomic clone
was sequenced as described above.
Southern and Northern blot hybridization
Genomic DNA of two different lines of homocloned Japanese
flounder, cloned lines 1 and 8, and two noncloned Japanese flounder
were isolated as previously reported (19). Total RNA was
prepared from either untreated PBLs or PBLs that had been stimulated
with LPS (500 µg/ml), Con A (50 µg/ml), or PMA (0.05 µg/ml) by
Trizol (Life Technologies, Rockville, MD). Five micrograms of total RNA
per lane was denatured at 65°C for 5 min in 50% formamide,
electrophoresed through a 1.5% agarose gel containing 6.6%
formaldehyde, and transferred to a nylon membrane (NEB, Beverly, MA).
Perfect RNA markers 0.210 kb (Novagen, Madison, WI) were used for
size marker of agarose gel electrophoresis. The probe was the full
length of a Japanese flounder TNF cDNA fragment and was labeled with
[
-32P]dCTP using a random primer labeling
kit (Takara Shuzo, Kyoto, Japan). Southern and Northern blot
hybridizations were done as described previously (21).
RT-PCR analysis
Total RNA was extracted from healthy Japanese flounder brain, head kidney, trunk kidney, liver, spleen, erythrocytes, leukocytes, skin, muscle, gill, heart, intestine, gonad, and LPS-treated leukocytes using Trizol (Life Technologies). The purified total RNA (10 µg) was reverse transcribed into cDNA using the AMV Reverse Transcriptase First-strand cDNA Synthesis kit (Life Science, Arlington Heights, IL). The final volume of the cDNA synthesis reaction was 25 µl. The reverse-transcribed sample (1 µl) was used in 50 µl of PCR mixture. The PCR primers used in this study were 5'-ggtttaaagtctcaaagtgc-3' and 5'-agttgactgtgagcatggtg-3'. The ß-actin primer set was used for a positive control of RT-PCR (22). PCR was performed with an initial denaturation step of 2 min at 95°C, and then 20 cycles were run as follows: 30 s of denaturation at 95°C, 30 s of annealing at 55°C, and 1 min of extension at 72°C. The reacted products were electrophoresed on a 2.0% agarose gel.
| Results |
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cDNA and
gene
From the expressed sequence tag analysis, we found a clone whose
amino acid sequence had significant identity to that of human TNF-
(data not shown). This clone was used as a probe for screening the cDNA
library and BAC genome library. The sequences of Japanese flounder TNF
cDNA and gene have been deposited in the DDBJ/GenBank/EMBL database
(accession numbers AB040448 and AB040449). The TNF cDNA consisted of
1217 bp, which coded 225 amino acid residues (Fig. 1
). There is an
N-glycosylation site on the 26th amino acid residue, but the
region is not included in the mature TNF. Fig. 2
shows an alignment of the amino acid
sequence of Japanese flounder TNF with the human TNF-
,
lymphotoxin-
, and -ß. The identities between Japanese flounder TNF
and previously reported mammalian TNF-
, lymphotoxin-
, and -ß
ranged from 20 to 35%. The Japanese flounder TNF shows 29 and 31%
amino acid identity to human TNF-
and lymphotoxin-
, respectively,
although the length of the amino acid sequence of the Japanese flounder
sequence is more similar to that of human TNF-
than to that of
lymphotoxin-
, whereas these two human proteins are themselves 30%
identical. The phylogenetic analysis suggests that the TNF-
and
lymphotoxin-
diverged after the divergence of mammals from teleosts
(Fig. 3
).
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9-kb
EcoRI DNA fragment from the isolated BAC clone and sequenced
entirely. The Japanese flounder TNF gene has a length of
2 kb and
consists of four exons and three introns (Fig. 5
gene. The lengths of the introns of the Japanese flounder
TNF gene are shorter than those of the human TNF-
gene. This is
especially true of the first intron of the Japanese flounder TNF gene,
which is one-sixth the length of the first intron in the human TNF-
gene. The Japanese flounder TNF gene has six simple dinucleotide
repeats, three in the 5'-upstream region and three in the 3'-downstream
region (Fig. 5
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As shown in Fig. 6
, mRNA from fresh
PBLs and unstimulated PBLs in the medium did not express TNF-
mRNA.
When the PBLs were stimulated with LPS, PMA, and a calcium ionophore,
or Con A, for 1 and 3 h, TNF mRNA expression was greatly induced.
However, when PBLs were stimulated with LPS, PMA, and the calcium
ionophore, or Con A, for 6 h, the expression level of TNF mRNA was
less than it was after 1- and 3-h stimulation. All RNA samples were
intact because rehybridization of the same membrane with a Japanese
flounder ß-actin cDNA probe (23) revealed a 1.4-kb band
in all lanes.
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| Discussion |
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30%) to human TNF-
and lymphotoxin-
, although the identity between human TNF-
and
lymphotoxin-
is
30%. The human TNF-
has two cysteine residues
in the mature protein region, which form a single disulfide bond,
whereas the lymphotoxin-
does not have a disulfide bond (Fig. 2
and lymphotoxin-
gene
were duplicated and evolved after mammals diverged from teleosts
(Fig. 3
gene. The protein coding regions of the
human lymphotoxin-
gene consist of three exons (15). We
speculate that the Japanese flounder TNF has a role similar to that of
the mammalian TNF-
based on similarities in the structures of the
genes, the lengths of the amino acid sequences, and the existence of
cysteine residues that form a disulfide bond.
There is no complete polyadenylation signal AATAAA in the
3'-untranslated region (UTR). However, TA-rich motifs (TTATTTAT)
within the mammalian TNF-
3'-UTR that were shown to influence
TNF-
mRNA half-life (6) and translational efficiency
(5) are also present in the Japanese flounder TNF cDNA
(Fig. 1
). Caput et al. (6) reported that the consensus
sequence TTATTTAT is present in the 3'-UTR of both human and mouse TNF
mRNAs, as well as the mRNAs encoding human lymphotoxin, human CSF,
human and mouse IL-1, human and rat fibronectin, and most of the
sequenced human and mouse IFNs. All of these mRNAs, except the
lymphotoxin mRNA, lack homology to the TNF mRNAs in the coding region
(6). It is particularly prevalent among mRNAs encoding
proteins related to the inflammatory response (6). Using
constructs in which the chloramphenicol acetyltransferase (CAT) coding
sequence is followed by varying segments of the TNF 3'-UTR, it was
possible to demonstrate that a downstream sequence present in the
TNF-
mRNA is sufficient to induce a more than 200-fold increase in
CAT synthesis in response to activation by endotoxin. The induction of
CAT activity is due to a marked enhancement of translational
efficiency, rather than to change in cytoplasmic mRNA concentration
(5). The Japanese flounder TNF gene has two complete
TTATTTAT sequences and some incomplete sequences of this type in the
3'-UTR. This suggests that the expression of Japanese flounder TNF is
regulated at the translation level.
The transcription of the mammalian TNF-
gene is also regulated by
various inducers, such as viruses, LPS, and PMA (24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34).
In contrast, the TNF-ß gene is not produced in response to LPS
(2). The human TNF-
gene contains NF-
B-binding
motifs. The NF-
B is thought to have a roll in constitutive high
level baseline expression of the human TNF-
gene (24).
Recently, a new regulation mechanism for TNF-
gene expression
involving LPS-induced transcription factor has been reported
(35). There are some potential NF-
B-binding motifs in
the 5' upstream region of the Japanese flounder TNF gene (Fig. 4
).
Interestingly, these regions are close to the simple dinucleotide
repeat sequences (Fig. 4
). These sequences might have a role in
controlling the expression of the Japanese flounder TNF gene.
Expression of the Japanese flounder TNF gene was induced by LPS, Con A,
and PMA, but the gene was not expressed in the cell culture medium
(Fig. 6
). Interestingly, the expression level of the TNF gene in
Japanese flounder PBLs after a 6-h incubation with LPS, Con A, or PMA
was lower than it was after 1 h and 3 h of incubation. The
highest induction were observed after a 1- and 3-h incubation with PMA
(Fig. 6
). The expression pathway of the Japanese flounder TNF gene
might respond quickly to some inducers of TNF gene transcription. These
results indicate that the Japanese flounder TNF gene, like the human
TNF-
gene, is regulated by various inducers, such as LPS, PMA, and
Con A. The present results also indicate that the Japanese flounder TNF
is more like mammalian TNF-
than mammalian lymphotoxin-
.
In conclusion, all of the characteristics of the cloned gene in this
study, i.e., its gene structure, amino acid sequence, and expression
pattern, are similar to those of human TNF-
, and thus indicate that
it is TNF-
. This is the first report of the TNF-
cDNA and gene
from a nonmammalian vertebrate.
| Footnotes |
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2 Address correspondence and reprint requests to Dr. T. Aoki, Department of Aquatic Biosciences, Tokyo University of Fisheries, Konan 4-5-7, Minato-ku, Tokyo 108-8477, Japan. ![]()
3 Abbreviations used in this paper: BAC, bacterial artificial chromosome; UTR, untranslated region. ![]()
Received for publication March 31, 2000. Accepted for publication July 24, 2000.
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